Thanks @Justin Cook!
Here is what I believe to be the correct revised diagram. If you would confirm I'd be very grateful. I'd also appreciate advice on the wire size I should use to connect the Neg terminals on batteries 1 and 3 to B2... It looks like the connections won't take much more than #18 - maybe less? That's all it takes?
Also, if you use the temperature sensor for the BMV 712, the wires from the temperature sensor completely replace the single power wire for the shunt, so the wire you have currently marked "to B1" goes away; the temperature sensor connects to your positive battery terminal, not the negative, and you connect the red wire from the temperature sensor to B1 on the shunt and the black wire goes to B2. Please do not connect the temperature sensor the way you currently have it drawn; as it's currently shown it will fry your PCB and likely the temp sensor as well.
One very important detail here is that your negative bus needs to be on the "load" side of your shunt, not the battery side. All negatives, including chassis grounds, charge source negatives, etc. etc. need to run to the "load" side of the shunt so that the BMV can see what's going on.
So: both negative cables (but nothing else) from your battery run directly to the "battery" side of the shunt; then connect your negative bus to the "load" side of the shunt and connect all other negatives to the bus.